This invention relates to lightly crosslinked linear olefinic polymer foam materials and processes for the preparation thereof.
Closed-cell branched ethylenic polymer foams may be manufactured by the process of extrusion foaming, wherein a normally solid thermoplastic ethylenic polymer resin, such as low density polyethylene, is melt processed and mixed under pressure with a volatile blowing agent to form a flowable gel. The gel is then passed through a shaping orifice or die opening into a zone of lower pressure. As the pressure is lowered, the volatile constituent of the gel vaporizes forming cells in the resin structure, which cools to a cellular foam material.
Linear olefinic polymers such as linear low density polyethylene, high density polyethylene, and polypropylene have several properties which make the use of these polymers in foams desirable. For example, these linear olefinic polymers have a higher modulus of elasticity, greater toughness, higher heat distortion temperature, and lower permeability to blowing agents than other branched olefins. However, previous attempts to produce low density foams of these linear olefins by an extrusion process have been unsuccessful. Linear polyolefins, when foamed by an extrusion process suffer from small foam cross section, high levels of open cells, flow instability, and a narrow range of foaming temperatures.
The exact cause of problems of foaming linear polyolefins is not believed to be known. However, it is generally believed that poor melt strength together with a sharp change in melt viscosity near the transition temperature makes extrusion foaming of linear polyolefins difficult. With these properties, it is difficult to control bubble expansion during extrusion and under conditions of heat and high shear stresses. This results in many broken bubbles (open cells) during expansion and consequent poor foams.
Previous efforts to cure these problems with tne foamability of linear polyolefins have centered around blending a linear polyolefin witn another olefin polymer having good extrusion foamability. For example, Park et al, U.S. Pat. No. 4,226,946, blended a linear polyethylene with a low density branched polyethylene to improve extrusion foamability of the linear polyethylene. Watanabe et al, U.S. Pat. No. 4,102,829, blended an ionomer resin with a linear polyethylene for the same purpose. However, such known methods offer only partial solutions because of one or more deficiencies in the range of operating parameters, cost of materials, and final foam properties.
It is also known that relatively lightly to moderately crosslinked thermoplastic polymers have melt properties that are suitable for foam expansion. However, such crosslinked polymers are difficult to process on conventional melt processing apparatus such as extruders because of flow instability. The prior art has generally not utilized crosslinked polymers on such apparatus because shear degradation of the polymer occurs during melt processing and extrusion.
Accordingly, there is a need in the art for linear olefinic polymer compositions and processes which can be utilized to foam or expand such polymers in conventional melt processing apparatus.